There is well known a vehicle provided with a mechanically operated step-variable transmission disposed between a drive power source and drive wheels. JP-2006-9942A discloses an example of this type of vehicle. This vehicle is a hybrid vehicle provided with an engine, planetary gear sets functioning as a differential mechanism, and first and second electric motors operatively connected to rotary elements of the planetary gear sets in a power transmittable manner. JP-2006-9942A also discloses a technique for calculating output torques of the first and second electric motors on the basis of their electric current values, estimating an input torque of the step-variable transmission on the basis of the calculated output torques of the first and second electric motors, and controlling transient hydraulic pressures to be applied to coupling devices of the step-variable transmission to implement its shifting actions, on the basis of the estimated input torque.
By the way, a high rate of increase of an output (both of power and torque) of the drive power source in the process of a shift-down action of the step-variable transmission may cause an excessively or unnecessarily large amount of increase of the output to be transmitted to the step-variable transmission in an inertia phase of the shift-down action, giving rise to a risk of an excessively high rate of rise so-called “racing” of an input shaft speed of the step-variable transmission due to an excess of the output, and a consequent risk of generation of a shifting shock of the step-variable transmission. In view of these risks, a control apparatus for a hybrid vehicle as disclosed in JP-2006-9942A is configured to implement a regenerative control of the second electric motor, so as to absorb an excess of the output transmitted to the step-variable transmission, for thereby restricting the rate of rise of the input shaft speed to reduce the risk of its racing and the shifting shock of the step-variable transmission. However, when an amount of electric power that can be regenerated by the second electric motor is limited, for example, when a maximum amount of electric power that can be stored in a battery is relatively small, the excess of the output of the drive power source cannot be sufficiently absorbed by the regenerative control of the second electric motor, so that there is still a risk of generation of the shifting shock of the step-variable transmission.